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Mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor formed by . It typically has a depth of ~ and rises about two kilometers above the deepest portion of an . This feature is where takes place along a . The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin. The production of new and oceanic results from upwelling in response to plate separation. The melt rises as at the linear weakness between the separating plates, and emerges as , creating new and lithosphere upon cooling. The first discovered mid-ocean ridge was the , which is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are traditionally called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries and the trace of the ridges across the ocean floor appears similar to the seam of a . The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about . Global system The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every , making it the in the world. The continuous mountain range is long (several times longer than the , the longest continental mountain range), and the total length of the oceanic ridge system is long. Description and , painted by Heinrich C. Berann (1977), showing the relief of the ocean floors with the system of mid-ocean ridges}} that spreads away from the ridge}} in , Iceland. The island is a sub-aerial part of the }} Morphology At the on a mid-ocean ridge the depth of the seafloor is approximately . On the ridge flanks the depth of the seafloor (or the height of a location on a mid-ocean ridge above a base-level) is correlated with its age (age of the where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate or A good approximation is that the depth of the seafloor at a location on a spreading mid-ocean ridge proportional to the square root of the age of the seafloor. The overall shape of ridges results from : close to the ridge axis there is hot, low-density mantle supporting the oceanic crust. As the oceanic plate cools, away from the ridge axis, the oceanic mantle (the colder, denser part of the mantle that, together with the crust, comprises the oceanic plates) thickens and the density increases. Thus older seafloor is underlain by denser material and is deeper. Spreading rate is the rate at which an ocean basin widens due to seafloor spreading. Rates can be computed by mapping of marine magnetic anomalies that span mid-ocean ridges. As crystallized basalt extruded at a ridge axis cools below s of appropriate iron-titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides. The orientations of the field preserved in the oceanic crust comprise a record of directions of the with time. Because the field has reversed directions at known intervals throughout its history, the pattern of in the ocean crust can be used as an indicator of age; given the crustal age and distance from the ridge axis, spreading rates can be calculated. Spreading rates range from approximately 10–200 mm/yr. Slow-spreading ridges such as the Mid-Atlantic Ridge have spread much less far (showing a steeper profile) than faster ridges such as the (gentle profile) for the same amount of time and cooling and consequent bathymetric deepening. Slow-spreading ridges (less than 40 mm/yr) generally have large s, sometimes as wide as 10–20 km (6.2–12.4 mi), and very rugged terrain at the ridge crest that can have relief of up to a . By contrast, fast-spreading ridges (greater than 90 mm/yr) such as the East Pacific Rise lack rift valleys. The spreading rate of the is ~ 25 mm/yr, while in the region, it is 80–145 mm/yr. The highest known rate is over 200 mm/yr in the on the East Pacific Rise. Ridges that spread at rates <20 mm/yr are referred to as ultraslow spreading ridges (e.g., the in the and the ). The spreading center or axis, commonly connects to a oriented at right angles to the axis. The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called s. At faster spreading rates the axes often display that lack connecting transform faults. The depth of the axis changes in a systematic way with shallower depths between offsets such as transform faults and overlapping spreading centers dividing the axis into segments. One hypothesis for different along-axis depths is variations in magma supply to the spreading center. Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults. Volcanism Mid-ocean ridges exhibit active and . The oceanic crust is in a constant state of 'renewal' at the mid-ocean ridges by the processes of seafloor spreading and plate tectonics. New magma steadily emerges onto the ocean floor and intrudes into the existing at and near rifts along the ridge axes. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of in the underlying . The upwelling solid mantle material exceeds the temperature and melts. The crystallized magma forms new crust of known as MORB for mid-ocean ridge basalt, and below it in the . Mid-ocean ridge basalt is a and is low in s. s fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers. Most crust in the ocean basins is less than 200 million years old, which is much younger than the age of the Earth. This fact reflects the process of lithosphere recycling into the Earth's mantle during subduction. As the oceanic crust and lithosphere moves away from the ridge axis, the in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the , which sits above the less rigid and viscous . Driving mechanisms Oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean . Two processes, and , are thought to be responsible for spreading at mid-ocean ridges. Ridge push refers to the gravitation sliding of the ocean plate that is raised above the hotter asthenosphere, thus creating a body force causing sliding of the plate downslope. In slab pull the weight of a tectonic plate being subducted (pulled) below an overlying plate at a drags the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push. A process previously proposed to contribute to plate motion and the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" due to deep (see image). However, some studies have shown that the ( ) is too plastic (flexible) to generate enough to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from and from observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi). On the other hand, some of the world's largest tectonic plates such as the and are in motion, yet only are being subducted in restricted locations such as the and , pointing to action by the ridge push body force on these plates. Computer modeling of the plates and mantle motions suggest that plate motion and mantle convection are not connected, and the main plate driving force is slab pull. Impact on global sea level Increased rates of (i.e. the rate of expansion of the mid-ocean ridge) has caused global ( ) sealevel to rise over very long timescales (millions of years). Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. This displaces the overlying ocean and causes sea levels to rise. can be attributed to other factors ( , ice melting, and creating y). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges. The high sealevel that occurred during the (144–65 Ma) can only be attributed to plate tectonics since thermal expansion and the absence of ice sheets by themselves cannot account for the fact that sea levels were 100–170 meters higher than today. Impact on seawater chemistry and carbonate deposition Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system. Hydrothermal vents at spreading centers introduce various amounts of , , , and other elements into the ocean, some of which are recycled into the ocean crust. , an isotope that accompanies volcanism from the mantle, is emitted by hydrothermal vents and can be detected in plumes within the ocean. Fast spreading rates will expand the mid-ocean ridge causing basalt reactions with seawater to happen more rapidly. The magnesium/calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released to seawater. Hydrothermal activity at ridge crest is efficient in removing magnesium. A lower Mg/Ca ratio favors the precipitation of low-Mg calcite polymorphs of ( s). Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and high-Mg calcite polymorphs of calcium carbonate ( s). Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas, meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown. The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading. History Discovery The first indications that a ridge bisects the basin came from the results of the British in the nineteenth century. Soundings from lines dropped to the seafloor were analyzed by oceanographers and and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. confirmed this in the early twentieth century. It was not until after , when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known. The , a ship of the of , traversed the Atlantic Ocean, recording echo sounder data on the depth of the ocean floor. A team led by and concluded that there was an enormous mountain chain with a rift valley at its crest, running up the middle of the Atlantic Ocean. Scientists named it the 'Mid-Atlantic Ridge'. Other research showed that the ridge crest was seismically active and fresh lavas were found in the rift valley. In addition, crustal heat flow was higher here than elsewhere in the Atlantic Ocean basin. At first, the ridge was thought to be a feature specific to the Atlantic Ocean. However, as surveys of the ocean floor continued around the world, it was discovered that every ocean contains parts of the mid-ocean ridge system. The traced the mid-ocean ridge from the into the early in the twentieth century. Although the first-discovered section of the ridge system runs down the middle of the Atlantic Ocean, it was found that most mid-ocean ridges are located away from the center of other ocean basins. Impact of discovery: seafloor spreading proposed the theory of in 1912. He stated: "the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot rising from depth". However, Wegener did not pursue this observation in his later works and his theory was dismissed by geologists because there was no mechanism to explain how s could plow through ocean , and the theory became largely forgotten. Following the discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. In the 1960s, geologists discovered and began to propose mechanisms for . The discovery of mid-ocean ridges and the process of seafloor spreading allowed for theory to be expanded so that it included the movement of oceanic crust as well as the continents. Plate tectonics was a suitable explanation for seafloor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major in geological thinking. It is estimated that along Earth's mid-ocean ridges every year of new seafloor is formed by this process. With a crustal thickness of , this amounts to about of new ocean crust formed every year. File:Deep_Sea_Vent_Chemistry_Diagram.svg|Oceanic ridge and deep sea vent chemistry File:Plates tect2 en.svg|Plates in the crust of the earth, according to the theory File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg|Seafloor File:Polarityshift.gif|A demonstration of magnetic striping List of mid-ocean ridges * * * * - an east-west trending mid-ocean ridge east of the eponymous islands between the and Cocos plates * * * * * * (Mid-Arctic Ridge) * * ** * * * ** Kolbeinsey Ridge (North of Iceland) ** Mohns Ridge ** Ridge (between Greenland and Spitsbergen) ** (South of Iceland) List of ancient oceanic ridges * * * * * * * References Category:Earth